Prior art for making nylon 6 on the basis of caprolactam is a process employing mono- or bifunctional carboxylic acids or amines as chain regulators. It has further been known for a long time to use chain regulators based on dicarboxylic acids (cf. U.S. Pat. No. 3,386,967 and DE Pat. No. 4,019,780).
In this known process, reaction partners are caprolactam, water as initiator and the abovementioned chain regulators. Additives such as, for example, pigments (TiO.sub.2 or others), light stabilizers or heat stabilizers may be included for specific applications. A characteristic feature of caprolactam polymerization is the formation of a reaction equilibrium. As well as the polymer, the melt includes about 10% of monomer (caprolactam) and cyclic oligomers. These low molecular weight constituents are dissolved out during chipmaking by an extraction with hot water. The extract water, which has an extractables concentration of up to 16%, depending on process specification, is generally concentrated by evaporation and the lactam separated from the oligomers by distillation. The oligomers are either removed from the process or depolymerized back to lactam. Lactam distillation and depolymerization are extremely energy-intensive.
The direct recycling of the concentrated extract water into the polymerization stage has hitherto been avoided for high grade applications of nylon 6 chip such as textile yarn or industrial yarn of high strength or film. The reason for this is the reduced quality such as reduced strength in the case of yarn or defective areas in film. Moreover, the spinning process is adversely affected by more broken ends and shorter spinning filter surface lives.
The underlying cause of these problems is in particular that the concentration of the cyclic oligomers of polycaprolactam, especially cyclic dimer, increases in the nylon 6 chip compared with nylon 6 chip produced without recycling of the concentrated extract water. The cyclic diner is particularly disadvantageous, since it has a very high melting point of 348.degree. C., which is far above the melting point of nylon 6 (220.degree. C.), and since it is partly present in the polymer melt in the form of undissolved particles or will rise to the surface in this form in the course of the processing into film and yarn and so cause yarn breakages or film defects.